1. Field of the Invention
The present invention relates to an ink, an ink cartridge, and an ink jet recording method.
2. Description of the Related Art
An ink to be used in an ink jet recording method has been required to additionally increase the optical density of an image to be recorded in recent years. Of the recording media on which images are to be recorded, various kinds of plain paper different from each other in ink permeability are available, and the difference affects image characteristics. In particular, the optical density of an image to be recorded on a recording medium having high ink permeability is liable to reduce.
To meet the above-mentioned requirement, there has been made a proposal concerning an increase in optical density of an image with an ink containing a self-dispersible pigment in which a functional group having high reactivity with calcium is bonded to the surface of a pigment particle (see Japanese Patent Application Laid-Open No. 2009-515007). Japanese Patent Application Laid-Open No. 2009-515007 describes a self-dispersible pigment having a phosphonic acid group. When an ink containing this self-dispersible pigment is stored, a phosphonic acid group may be detached to reduce the storage stability of the ink depending on the structure of a functional group of the self-dispersible pigment. To cope with such problem, there has been made a proposal concerning the use of a self-dispersible pigment having a structure capable of suppressing the detachment of a phosphonic acid group (see Japanese Patent Application Laid-Open No. 2011-515535).
In addition, there has been made a proposal concerning improvements in optical density, uniformity, scratch resistance, and the like of an image to be recorded with an ink containing two kinds of pigments different from each other in peak shape in a particle size distribution (see Japanese Patent Application Laid-Open No. 2007-217695).